The Telegraph's letters have had items on this. Suggestions include Glyphosate, diluted caustic soda, salt in warm water, and bleach on paths and drives. A quick search of glyphosate turns up several weedkillers.

I use ordinary Table Salt to 'Spot Weed'I cut the top off the weed and pour about a level teaspoonful out of a salt pot directly onto the root left in ground.Very effective and cheap, and no danger to children, animals etc.

Salt may be the answer to spot weed killing, but surely there are lots of plants which will not grow near the seaside. So spraying large areas of land would not be helpful. I suppose a spot of salt on a dandelion's innards would kill it and the salt would wash away and harm nothing except the occasional early bird's breakfast. By the way who relegated the beautiful dandelion to the weed category. I think it is a very nice flower and would be easy to grow anywhere without having to have "green fingers".